For those of you that grow citrus in ground in marginal climate zones, at what forecast temperatures do you implement active freeze protection?

I live on the 9a/8b line and do the following, my more cold sensitive citrus (Meyer's and Cara Cara) get C7 Christmas lights and covering with a light sheet any time the forecast temperature drops below freezing, which may be a bit conservative, but all it takes is the forecast to be significantly wrong once to loose them, already once this year we had a forecast calling for a low of 33F and had an actual low of 26F. My young more cold hardy Satsuma's get covered when the forecast calls for a low below 25-26, and my large mature Satsumas (12-15 ft tall 15-20 ft wide) only get covered and heat lamps when the forecast calls for temperatures below 20 degrees, which averages being once every 5 years or so.

The smaller trees all have C7 style Christmas lights on them throughout the winter which are plugged into thermo-cube switches that turn on at 35F and off at 45F, which without covering only adds minimal protection, but minimal is better than one.

I cover my satsumas, kumquat, grapefruit trees and use space heaters with thermocubes under each plastic cover whenever it's supposed to get down to 32 degrees , because my trees have fruit on them all winter long, and a freeze here can last for days, and it can easily be 10 degrees colder or warmer on top of this mountain than it may be down in Asheville, NC. Like you, I've put too much time, money, love into my citrus over the years to lose them over a single weather event. Citranges and poncyrus, yeah, no worries, they can take it, but real citrus don't like freeze. I think your smart to protect your valuable trees. Good for you.